1. Field of the Invention
The present invention relates to a lockup device of a hydraulic torque transmission device. Specifically, the present invention relates to a lockup device of a hydraulic torque transmission device including a front cover supplied with a torque, an impeller, which is fixed to the front cover to form a fluid chamber filled with working fluid, and a turbine opposed to the impeller. More specifically, the invention relates to the lockup device, which is arranged between the front cover and the turbine to couple mechanically the front cover and the turbine together.
2. Background Information
A torque converter is a kind of hydraulic torque transmission device. The torque converter is internally provided with three kinds of vane wheels (impeller, turbine, and stator), and is configured to transmit a torque by circulation of working fluid kept therein. The impeller is fixed to a front cover coupled to an input rotary member (i.e., rotary member on an input side). When the impeller rotates, the working fluid flows from the impeller toward the turbine to rotate the turbine. Thereby, the turbine provides torque to an input shaft. This kind of torque converter is often provided with a lockup device.
The lockup device is arranged between the turbine and the front cover, and is configured to couple mechanically the front cover to the turbine to transmit directly transmitting the torque therebetween. Japanese Laid-Open Patent Publication NO. S63-72968 discloses a lockup device structure having three friction surfaces. Japanese Laid-Open Patent Publication No. S63-72968 is hereby incorporated by reference.
The lockup device having three friction surfaces primarily includes a damper mechanism, first and second friction plates, and a piston. The damper mechanism has a hub flange that rotates together with a turbine, retaining and clutch plates, which are rotatable with respect to the hub flange, and torsion springs that elastically couple the hub flange to the retaining and clutch plates in a rotating direction. The torsion springs are held by the retaining and clutch plates. The first friction plate is unrotatably and axially movably engaged with the retaining and clutch plates. The second friction plate is unrotatably and axially movably engaged with a radially outermost portion of the front cover. The piston is fixed to the retaining and clutch plates by rivets. The piston is axially and closely arranged on and to a turbine side of the second friction plate. Further, the piston can be moved axially together with the damper mechanism by changes in pressure of the working fluid.
In this lockup device, when the working fluid is discharged from a space on an front cover side (i.e., a side axially opposed to the front cover) of the piston, hydraulic pressure in a space on a turbine side (i.e., a side axially opposed to the turbine) of the piston becomes higher than that in the space on the front cover side so that the piston moves axially toward the front cover. Thereby, the piston pushes the second friction plate, which in turn pushes the first friction plate so that the first friction plate is pressed against the friction surface of the front cover, and frictional coupling is achieved. Therefore, the torque of the front cover is transmitted from the friction surface and the second friction plate through the first friction plate and the piston to the retaining and clutch plates, and further the torque is transmitted to the hub flange through the torsion springs, and is output to the turbine. This arrangement will be referred to as “lockup ON.”
In the lockup ON state, when the working fluid is supplied into the space on the front cover side of the piston, the pressure in the space on the front cover side of the piston increases to move the piston axially toward the turbine. Thereby, the piston no longer presses the second friction plate axially against the front cover, and the first friction plate is no longer pressed against the friction surface of the front cover so that the frictional coupling is released. Therefore, the torque of the front cover is not transmitted from the friction surface and the second friction plate through the first friction plate and the piston to the retaining and clutch plates, but is transmitted from the impeller to the turbine by driving fluid. This will be referred to as “lockup OFF.”
In the above lockup device having the three friction surfaces, the piston is moved axially by the changes in hydraulic pressure. Therefore, a situation may occur such that the pressure in the space on the front cover side of the piston becomes higher than the pressure in the space on the turbine side of the piston, and the piston is axially pulled toward the turbine, as is done in the case when the torque converter is reversely driven. In the above case, when an operation is performed to attain the lockup ON state, the piston cannot rapidly move axially toward the front cover, resulting in low response property problems. In the operation of attaining the lockup OFF state, the piston to be moved axially toward the turbine is in such a state the pressure in the space on the turbine side of the piston is higher than that in the space on the front cover side of the piston. This likewise results in low response property problems.
Further, in the conventional lockup device having the three friction surfaces, the second friction plate is axially movable so that a clearance cannot be ensured between a pushing portion of the piston and the second friction plate in the lockup OFF state. Thus drag torque problems are liable to occur.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved lockup device of a hydraulic torque transmission device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.